"Drill steels" is the term applied to hollow tubes which are used to transmit rotating power from a power source to a drill bit. The steels telescope together to provide a drill train as long as desired for a particular application. The telescopic connection is usually achieved by an ensmalled portion or male end at one end of a steel which fits into a recess at the other end of a corresponding drill steel. The so-called male end or insertion end has been applied to the drill steel by placing and joining an insert part way into the drill steel tube. The insert is joined to the exterior of the drill steel by plug welds 180.degree. apart made in holes or slots premachined in the drill steel. Thus, a portion is extending from the tube and insertable into an adjoining tube.
In U.S. Pat. No. 4,299,510, issued Nov. 10, 1981, there is disclosed a drill steel which is formed in a two-step process which eliminates the need for any welding and insures a tighter connection for the male end. In addition, the elimination of welds avoids the creation of as-cast and heat-affected zones in the microstructure of the weldments, and also reduces the possibilities of detrimental residual stresses and notch-type defects often associated with weldments.
The present invention is directed to an improved drill steel insert in the form of an upset forging formed in a one-hit hot forming process. By changing the lead end of the insert from the blunt heavy section walls, as shown in the referenced patent, to a thinner necked-down shape, a single strike in the forming process is all that is required to establish the insert in the drill steel and at the same time to enlarge the drill steel itself.
Thus, an object of the invention is the provision of a drill steel with a thinner necked-down portion which is effective in its function to receive and retain a lead insert and yet can be made in a less expensive manner. It is a further object to provide a one-step method in which a permanent insert performs forming and assembling functions in the process of manufacture.
Other objects and features of the invention will be apparent in the following description and claims in which the invention is described and details provided to enable persons skilled in the art to utilize the invention, all in connection with the best mode presently contemplated for the practice of the invention.